A rubber modified styrene-containing resin has a good processability, a high impact strength, and a good appearance. Accordingly, the resin has been widely applied to electric appliances and office supplies. In case that a rubber modified styrene-containing resin is applied to personal computers, facsimiles, and the like which emit heat, flame-retardant property should be given to the resin due to its combustibility.
A widely known method for providing flame retardancy to resin compositions is the addition of halogen-containing compounds and antimony-containing compounds to a rubber modified styrene-containing resin to provide flame-retardant property. The halogen-containing compounds used in conventional methods are, for example, polybromodiphenyl ether, tetrabromobisphenol A, epoxy compounds substituted by bromine, chlorinated polyethylene, etc. An antimony trioxide and antimony pentaoxide are commonly used as antimony-containing compounds. The methods for improvement of flame-retardant property by applying halogen- and antimony-containing compound have advantages of being a convenient method of obtaining flame-retardant property and no degradation of the physical properties. However, the disadvantages are that the halogen-containing compound results in the corrosion of the mold itself by the hydrogen halide gases released during the molding process. Hydrogen halide gas is extremely harmful due to the toxic gases liberated during combustion of articles containing halogen-containing flame retardants. Since polybromodiphenyl ether compounds that are mainly used for a halogen-containing flame retardant can produce toxic gases such as dioxin or furan during combustion, flame retardant compositions which are not prepared with halogen-containing compound have become important in this field.
It is known to use phosphorus or nitrogen compounds as halogen-free flame retardants in resin compositions. However, usage of only phosphorus compound deteriorates heat resistance of a rubber modified styrene-containing resin and does not impart sufficient flame retardancy. Generally, when a rubber modified styrene-containing resin such as acrylonitrile-butadiene-styrene (ABS) is burned, char is scarcely produced due to decomposition and vaporization in most parts (Journal of Applied Polymer Science, 1998, vol. 68, p. 1067). Therefore, to impart an effect of flame retardancy, it is necessary to add a char-forming agent to a resin composition, which plays a role to prohibit entrance of oxygen and emission of fuel by forming char on the surface of rubber with three-dimensional carbon chain bonds under combustion.
Japanese Patent Laid-open No. 7-4849 1 discloses a flameproof thermoplastic resin, made of adding a novolac phenolic resin and a phosphoric acid ester in to a thermoplastic copolymer resin composed of a rubber copolymer and an aromatic vinyl monomer. It was found that in order to obtain a good property of flame retardancy, a phenolic resin as a char-forming agent and a phosphoric acid ester compound as a flame retardant should be added in a lot of amount. However, in this case, the heat resistance of the resin composition is dropped suddenly. Therefore, many studies have been done to prepare a flameproof resin composition which does not contain phenol resin but shows a good flame retardancy and heat-resistant.
U.S. Pat. No. 4,360,618 discloses that the impact strength and heat-resistance of a resin composition was improved by mixing SAN resin or ABS resin copolymer with 2 to 8% by weight of acrylonitrile to improve compatibility of polyphenylene ether and styrene-containing resin with acrylonitrile. U.S. Pat. No. 4,578,423 discloses that the mechanical properties of a resin composition with a high content of acrylonitrile were maintained by blending 10 to 90% by weight of polyphenylene ether with 10 to 90% by weight of SAN resin or ABS resin copolymer with 8 to 17% by weight of acrylonitrile (standard deviation of the weight % of acrylonitrile about the mean value is at least 3.5%). But, if the content of acrylonitrile was higher than that of polyphenylene ether resin, the compatibility of styrene-containing resin and polyphenylene ether deteriorated, so the mechanical property of the resin deteriorated.
Commonly owned Ser. No. 10 011,056, herein incorporated by reference, discloses flame retardant compositions that contain rubber modified styrene-containing resin, polyphenylene ether resin, a styrene/acrylonitrile copolymer having a certain nitrile content, and phenolic resin with aromatic phosphate esters having melting points above 90° C.
In Ser. No. 10/052,205 filed on Jan. 16, 2002, Ser. No. 09 473,607 and Ser. No. 09 473,553 and Ser. No. 09 580,907, all herein incorporated by reference, the present inventors disclosed blends of rubber modified styrene-containing resins having a particular nitrile content and polyphenylene ether resins wherein their compatability is improved by the use of a compatabilizer having a particular nitrile content. Unlike the prior art compositions discussed previously, the compositions disclosed in these applications do not require phenolic resin to obtain good mechanical properties and flame retardance. On the contrary, the presence of even relatively minor amounts of phenolic resin or red phosphorous in these compositions has adverse effects on properties such as the heat stability and/or the color of the composition.
The present inventors have discovered that the use of a rubber modified styrene-containing copolymer resin with about 15 to 40% by weight, excluding rubber, of a nitrile monomer such as acrylonitrile having a particular nitrile content distribution with polyphenylene ether resin and an aromatic phosphoric acid ester flame retardant produces resin compositions with high impact strength and flame retardancy. The compositions according to the present invention are substantially free of phenolic resin, red phosphorous, silicone resin and other components that are required by the prior art compositions.